It's a "bad situation" because he made it one
He was brought here and giftwrapped a starting spot, with Ainge hoping he would be able to showcase him for a trade. He started all through preseason and through the first 3 or 4 games in the season but got sent to the bench because of how awful he was.
It has little to do with Stevens wanting to play bigs who shoot 3's. Last year he started Brandon Bass & Tyler Zeller, neither shoot 3's. This year he opened the season up with Lee & Zeller, neither shoot 3's. Amir Johnson has been starting for the vast majority of the season and if we're calling him a stretch big then the standards are extremely low.